1. Field of the Invention
The present invention relates to internal combustion engines and, more particularly, to a two-stroke, over expanded compression ignition (CI) engine cycle designed to directly lower the temperature of combustion in order to reduce NOx formation. In addition to reducing combustion temperatures, the cycle extends/prolongs the period during the cycle that combustion occurs allowing for more complete burning of the fuel charge, which also reduces CO and HC emissions.
2. Background of the Prior Art
In the quest to develop a better internal combustion engine, satisfying emissions requirements is paramount. Potential gains in power and/or efficiency are immaterial, unless a new engine design is able to meet such requirements in a commercially feasible way. Of the three primary noxious emissions (NOx, CO and HC), developing ways to reduce nitrogen oxide (NOx) emissions is perhaps the most vexing. Moreover, under current engine operating cycles, potential solutions that address NOx emissions tend to exacerbate carbon monoxide (CO) and hydrocarbon (HC) emissions.
NOx formation during combustion is caused by high combustion temperature. Exhaust gas recirculation (EGR) is used in virtually all commercial automobile internal combustion engines in order to reduce combustion temperature so as to reduce NOx emissions. Similarly, homogeneous charge compression ignition (HCCI) technology, which is garnering increasing attention as a potential means to control NOx emissions, relies on autoignition of a homogeneous lean charge, which results in a uniform temperature distribution following combustion without significant temperature gradient, thereby reducing the overall maximum combustion temperature. The level of NOx produced during combustion can be controlled by limiting combustion temperature to a selected temperature below which NOx does not form in unacceptable levels.
In traditional diesel engines, the high temperature of the early burned fuel-air mixture is the primary culprit in NOx formation. More specifically, combustion is initiated by injecting fuel into compressed air and the temperature rises due to the burning of the fuel. Importantly, the combustion of the fuel results in expansion of the burning gases thereby causing a rapid increase in pressure. This rapid increase in pressure following combustion causes an additional increase in temperature of the already burned gas. The cumulative increase in temperature that results from the burning of the fuel-air mixture, plus the additional increase caused by the post-burning compression, is referred to herein as the “post-combustion temperature.” In a traditional diesel engine, the post-combustion temperature exceeds the threshold temperature at which unacceptable levels of NOx are formed.